Random access burst transmission scheme

ABSTRACT

An apparatus for transmitting and receiving signals in a wireless telecommunication system and to a method for transmitting random access bursts in such an apparatus. The apparatus includes a transmitter for transmitting random access bursts, a receiver for receiving acknowledgement information confirming a successful transmission of a random access burst, a decision unit for deciding if acknowledgement information has been received, and a control unit for randomly choosing a time point for an attempt to retransmit said random access burst from an interval if no acknowledgement information has been received after a random access burst had been transmitted.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.09/521,394, filed Mar. 8, 2000, and is based upon and claims the benefitof priority from prior European Patent Application No. 99 104 766.3,filed Mar. 10, 1999, the entire contents of each of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an apparatus for transmitting andreceiving signals in a wireless telecommunication system and a methodfor transmitting random access bursts in such an apparatus.Particularly, the present invention relates to a backoff scheme for thetransmission of random access bursts in a mobile station of a wirelesstelecommunication system.

2. Description of the Related Art

In wireless telecommunication systems, as for example the GSM system,the mobile station uses the random access channel (RACH) to send randomaccess bursts to acquire services from the network. If for example amobile station of a mobile cellular system needs a data channel totransmit user data, it sends a random access burst over the randomaccess channel to the selected base station. The base station receivinga random access burst sends an acknowledgement or a confirmation messageto the mobile station to indicate, that the transmission of the randomaccess burst from the mobile station to the base station has beensuccessful. The confirmation information might be the access grantinformation sent by a base station to indicate that the requestedservice is granted. After a successfully transmitted random accessburst, the base station allocates the respective requested service, forexample a user data channel, to the mobile station.

The transmission of a random access burst from a mobile station to abase station is contention based and collisions can occur. If forexample several mobile stations within one cell of the telecommunicationsystem simultaneously send random access bursts, collisions can occurand the random access bursts can not be correctly received and confirmedby the base station. Having sent a random access burst to a basestation, the mobile station checks if an acknowledgement informationfrom the base station has been received. If an acknowledgementinformation has not been received after a predetermined time period, themobile station decides that the transmission attempt of the randomaccess burst has not been successful. After such a failed random accessburst transmission attempt, a backoff mechanism determines, when a newtransmission of the random access burst shall take place. The same istrue after a retransmission of a random access burst and not only forthe first transmission attempt.

Different kinds of backoff mechanisms are known in the prior art. Forexample, in a so-called bayesian backoff scheme, the network side, i.e.the base station, estimates, if the random access channel has beenoccupied or not, if collisions occurred or if a random access burst hasbeen successfully transmitted. Estimation information about the resultof this estimation is sent to the mobile stations, so that the mobilestations can set their backoff mechanisms depending on the estimationinformation. Another known possibility is a detection of the collisionsof competing random access bursts at the user equipment side. The knownsystems require much downlink resources, since the base station has toregularly transmit estimation information to the mobile stations or arevery complex because collision detection is needed.

The U.S. Pat. No. 5,544,196 discloses an apparatus and a method forreducing message collision between mobile stations simultaneouslyaccessing the base station in a wireless telecommunication system.Hereby, a mobile station inserts a random delay between successiverandom access burst transmissions if it does not receive anacknowledgement after a predetermined time period. The random delay isset by randomly choosing a time point from a predetermined timeinterval. Thus, the time interval, from which a new time point for aretransmission of a random access burst is choosen, is fixedly preset.This relatively simple backoff mechanism is also used in the GSM system.Particularly in cases, in which the random access channel is veryoccupied and a lot of collisions occur, this known system is veryineffective, since a lot of random access burst transmission attemptsare performed with a very low probability of a successful transmission.

SUMMARY OF THE INVENTION

The object of the present invention is therefore to provide an apparatusfor transmitting and receiving signals in a wireless telecommunicationsystem and a method for transmitting random access bursts in anapparatus for transmitting and receiving signals in a wirelesstelecommunication system, which provide a more effective backoff schemefor the transmission of the random access bursts.

This object is achieved by an apparatus for transmitting and receivingsignals in a wireless telecommunication system with transmitting meansfor transmitting random access bursts, receiving means for receivingacknowledgement information confirming a successful transmission of arandom access burst, decision means for deciding if acknowledgementinformation has been received, and control means for randomly choosing atime point for an attempt to retransmit said random access burst from aninterval if no acknowledgement information has been received after arandom access burst had been transmitted, characterized in that saidcontrol means sets the length of said interval depending on the numberof unsuccessful attempts to transmit a random access burst.

The above object is further achieved by a method for transmitting randomaccess bursts in an apparatus for transmitting and receiving signals ina wireless telecommunication system including the steps of transmittinga random access burst, deciding, if an acknowledgement informationconfirming a successful transmission of said random access burst hasbeen received, and randomly choosing a time point for an attempt toretransmit said random access burst from an interval if noacknowledgement information has been received, characterised by settingthe length of said interval depending on the number of unsuccessfulattempts to transmit a random access burst.

SUMMARY OF THE INVENTION

The present invention thus provides a backoff scheme for retransmissionof random access burst, which is very effective, flexible and does notrequire a high complexity. Particularly, the present invention does notrequire a system to detect or estimate collisions of random accessbursts at the network side but enables at the same time a flexibleadaptation of the backoff scheme to the current random access channelsituation. By setting the length of the interval, from which the timepoint for the retransmission is randomly choosen, depending on thenumber of previous unsuccessful attempts, the number of unsuccessfulattempts until a successful transmission of a random access burst can bereduced. Thereby, the performance is enhanced and the system stabilityis improved. At the same time a high and effective throughput of randomaccess bursts on a random access channel can be achieved. Although thepresent invention can advantageously be applied in mobile stations of awireless telecommunication system, it might also be advantageous forterminals of other systems.

Advantageously, the interval length is set longer when the number ofunsuccessful attempts increases. Hereby, in situations of heavy trafficon the random access channel and a large number of collisions, the timepoint for a retransmission is postponed and the probability for asuccessful retransmission is significantly increased. Hereby, theinterval length is advantageously depending exponentially on the numberof unsuccessful attempts.

Advantageously, the control means randomly chooses the time point for aretransmission attempt after a predetermined offset time has elapsed.After transmission or retransmission of a random access burst it isdetermined only after said predetermined offset time has passed if anacknowledgement information, e.g. from a selected base station, has beenreceived.

Further, the control means can only choose a time point for aretransmission attempt if it is determined that the number ofunsuccessful attempts is lower than a predetermined maximum number ofunsuccessful attempts. By setting a maximum number of attempts it can beavoided that an infinite number of attempts is performed. Sinceaccording to the present invention the length of the interval depends onthe number of unsuccessful attempts, the probability is reduced, thatthe maximum number of retransmissions is reached.

The concept of the present invention is further advantageous in that itallows to allocate different priorities for the service requeststransmitted with the random access bursts. Hereby, the interval lengthcan be set depending on different priority classes of the random accessbursts. Advantageously, the interval length is thereby set shorter forhigher priority classes. This allows to provide a very small mean timeuntil a successful transmission of a random access burst for highpriority service requests, whereby random access bursts of lowerpriority classes consequently have a higher mean time until a successfultransmission.

Further advantageously, the interval is represented by a series ofnumbers representing time slots in the wireless telecommunicationsystem.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following description, the present invention is explained in moredetail on the basis of a preferred embodiment relating to the encloseddrawings, in which

FIG. 1 shows a flow chart for explaining the back-off method of thepresent invention, and

FIG. 2 shows a bloc diagram of a terminal of a wirelesstelecommunication system incorporating a back-off mechanism structureaccording to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a flow chart of a method for transmitting random accessbursts in a terminal of a wireless telecommunication system according tothe present invention. In step S1, a first transmission of a randomaccess burst generated in the terminal is sent. If, e.g., a mobilestation of the wireless telecommunication system needs a channel totransmit user data, a service request in form of a random access burstis transmitted to the selected base station. In case that the basestation successfully receives the random access burst, it generates andtransmits an acknowledgement information back to the mobile station. Theacknowledgement information can e.g. be an access grant information orthe like. In case that the mobile station successfully receives theacknowledgement information, it decides in a step S2, that the firsttransmission of the random access burst has been successful.Consequently, no retransmission attempts are necessary and the procedureis terminated in a step S3. A base station can transmit together with orintegrated in the acknowledgement information further information on theallocation of the requested service, e.g. on the channel to be used forthe transmission of user data.

After the transmission of a first random access burst in step S1, theapparatus, e.g. the mobile station, decides, if an acknowledgementinformation had been received or not. In case that an acknowledgementinformation has not been received after a predetermined offset timeperiod has elapsed, the decision is negative and an unsuccessfultransmission is assumed. In case an acknowledgement information isreceived within said predetermined offset time period, the decision ispositive and a successful transmission is assumed.

In case that the decision in step S2 is negative, the procedure goes tostep S4, in which the number of unsuccessful attempts is accumulated. Inthe next step S5 it is determined, if the number of unsuccessfulattempts has reached a predetermined maximum number of attempts. If thisis the case, the procedure is terminated in step S6. In case that theaccumulated number of unsuccessful attempts has not yet received thepredetermined maximum number of attempts, the procedure goes to step S7.In step S7, a new time interval is set, from which a new time point forthe next transmission of the random access burst is choosen in step S8.Hereby, the new time interval set in step S7 is calculated on the basisof the accumulated number of unsuccessful previous attempts. The timeinterval is thereby set by calculating a number of time slots dependingon the accumulated number of unsuccessful attempts. The time interval isthus represented by a series of numbers {1, 2, . . . ,MW+S.times.B.sup.PO+NF}, whereby each number represents a number of timeslots as the basic unit. The time slots are thereby the basic time slotsof the respective wireless telecommunication system. The factor MW isthe minimum length of the time interval, the factor S is a scalingfactor, the factor B is a base factor, the factor PO is a power offsetfactor and the factor NF is the number of failures. The number offailures is hereby the accumulated number of unsuccessful attemptscalculated in step S4. All factors are non-negative integers, wherebythe factor NF varies from attempt to attempt. The other factors caneither be preset or varied more slowly on the basis of other systemparameters.

In step S8, a new time point for the next transmission or theretransmission of the random access burst is randomly choosen from thetime interval set in step S7. Hereby, the new time point is randomlydrawn on the basis of the uniform probability distribution. As can beseen from the above formula, the time between transmissions is increasedwith an increasing number of unsuccessful attempts or failures.Particularly due to the exponential dependency, the time interval fromwhich the new time point is randomly choosen is strongly extended sothat the probability of future unsuccessful attempts, e.g. due tocollisions, is significantly reduced. Thus, the overall system stabilityis strongly enhanced.

The factor MW which determines the minimum width of the time intervalcan e.g. be set to ten time slots, so that a certain minimum timeinterval is assured. The factors S, B and PO can be used to supportdifferent priority classes for the random access bursts. Thereby, thesefactors are preset so that the time interval is shorter for higherpriority classes. If, e.g. four different priority classes for fourdifferent classes of service requirements and consequently random accessbursts are assumed, the factors can have different values for thedifferent priority classes. Hereby, it is sufficient to vary e.g. onlythe scaling factor S for the different priority classes. For the highestpriority class, the factors could be set as follows: S=1, B=2, PO=0. Forthe second priority class, the following values could be used: S=4, B=2,PO=0. For the third priority class, the factors could be set as follows:S=8, B=2, PO=0. For the fourth and lowest priority class, the factorscould be set as follows: S=12, B=2 and PO=0. Simulations have shown,that it is sufficient and thus recommendable to vary only the factor Sso that the calculation scheme is simplified.

After step S8, a retransmission of the random access burst is performedat the randomly choosen time point. Thereby, the time interval set instep S7 determines the number of time slots starting from the end of thepredetermined offset time, after which it is determined in step S2, ifan acknowledgement information had been received or not. The absolutetime point for a retransmission of a random access burst is thus givenby the predetermined offset time plus the time point choosen in step S8.As can be seen from the above example, only the scaling factor S isadvantageously varied for different priority classes. The base parameterB and the power offset parameter PO remain unchanged, whereby theseparameters can also be changed to adapt the backup mechanism to slowlyvarying system conditions.

In FIG. 2, a bloc diagram of an apparatus for transmitting and receivingsignals in a wireless telecommunication system according to the presentinvention is shown. The shown apparatus 1 can e.g. be a mobile stationof a cellular mobile system and is adapted to perform the steps shown inand explained in relation to FIG. 1. Thereby, the bloc diagram of FIG. 2only shows the elements which are important in relation to the presentinvention. Other elements which are necessary to operate the shownapparatus 1 in a corresponding telecommunication system, likemodulators, demodulators, interleavers, microphone, loudspeaker and soforth are not shown for the sake of clarity.

The apparatus 1 comprises an antenna 2 for receiving and transmittingsignals in the wireless telecommunication system. The antenna 2 isconnected to a transmitting means 3 and a receiving means 4. Thetransmitting means 3 serves for transmitting signals and can e.g.comprise a corresponding high frequency portion. The receiving means 4serves for receiving signals and comprises a corresponding highfrequency portion for downconverting the received high frequency signalsto the base band. The receiving means 4 is connected to a decision means5. The transmitting means 3 is connected to a random access channelgeneration means 7, in which random access bursts are generated. Boththe random access generation means 7 and the decision means 5 areconnected to a control means in form of a central processing unit 6.

In case that a service request or the like has to be made from theapparatus 1 to another apparatus of the telecommunication system, thecentral processing unit 6 gives a corresponding signal to the randomaccess generation means 7 to generate the necessary random access burst.The random access burst is then transmitted by the transmitting means 3.The decision means 5 checks if a corresponding acknowledgementinformation has been received within a predetermined offset time periodby the receiving means 4. The central processing unit 6 thereforeinforms the decision means 5 on the time point of a transmission of arandom access burst by the transmitting means 3 so that the decisionmeans 5 is able to compare the actual time given by a timing generatorwith the predetermined offset time period starting from the time pointof the transmission of a random access burst. In case that anacknowledgement information is received by the receiving means 4 withinthe predetermined offset time period, no retransmission of a randomaccess burst is necessary. In case that no acknowledgement informationhas been received after the predetermined offset time has elapsed, thecontrol means 6 randomly chooses a time point for an attempt toretransmit a random access burst from a time interval. The time intervalis thereby set by the control means 6 depending on the number ofprevious unsuccessful attempts to transmit the random access burst. Thecontrol means 6 thereby sets the time point for the next attempt likedescribed in relation to steps S7 and S8 of FIG. 1.

Therefore, the present invention provides a backoff scheme fortransmitting random access bursts in a telecommunication apparatus witha very good performance and with a very high stability as compared toknown systems. Particularly, the throughput is still considerably higheven for high loads, e.g. in regions of congestion. Further, no backoffspecific resources on the downlink channel are required and thedetection of collisions on the network side is not necessary.

1. A method for transmitting random access bursts in an apparatus fortransmitting and receiving signals in a wireless telecommunicationsystem, comprising: transmitting a random access burst; deciding ifacknowledgement information confirming a successful transmission of saidrandom access burst has been received; randomly choosing a time pointfor an attempt to retransmit said random access burst from an intervalif no acknowledgement information has been received, wherein differentpriority classes are supported for the random access bursts; and settingthe length of said interval depending on at least two parameters,wherein a first parameter indicates the number of unsuccessful attemptsto transmit a random access burst and a second parameter indicates thepriority class, and said second parameter is chosen so that saidinterval is set shorter for higher priority classes.
 2. The method fortransmitting random access bursts in an apparatus for transmitting andreceiving signals in a wireless telecommunication system according toclaim 1, wherein said interval length is set longer when the number ofunsuccessful attempts increases.
 3. The method for transmitting randomaccess bursts in an apparatus for transmitting and receiving signals ina wireless telecommunication system according to claim 2, wherein saidinterval length depends exponentially on the number of unsuccessfulattempts.
 4. The method for transmitting random access bursts in anapparatus for transmitting and receiving signals in a wirelesstelecommunication system according to claim 1, wherein said time pointfor a retransmission attempt is randomly chosen after a predeterminedoffset time has elapsed.
 5. The method for transmitting random accessbursts in an apparatus for transmitting and receiving signals in awireless telecommunication system according to claim 1, wherein saidtime point for a retransmission attempt is only chosen if it isdetermined that the number of unsuccessful attempts is lower than apredetermined maximum number of unsuccessful attempts.
 6. A method fortransmitting random access bursts in an apparatus for transmitting andreceiving signals in a wireless telecommunication system, comprising:transmitting a random access burst; deciding if acknowledgementinformation confirming a successful transmission of said random accessburst has been received; randomly choosing a time point for an attemptto retransmit said random access burst from an interval if noacknowledgement information has been received, wherein differentpriority classes are supported for the random access bursts; and settingthe length of said interval depending on the number of unsuccessfulattempts to transmit a random access burst and on the respectivelypreset priority class, wherein said interval is shorter for higherpriority classes.
 7. A method for transmitting random access bursts inan apparatus for transmitting and receiving signals in a wirelesstelecommunication system, comprising: transmitting a random accessburst; deciding if acknowledgement information confirming a successfultransmission of said random access burst has been received; randomlychoosing a time point for an attempt to retransmit said random accessburst from an interval if no acknowledgement information has beenreceived, wherein different priority classes are supported for therandom access bursts; and setting the length of said interval dependingon the number of unsuccessful attempts to transmit a random access burstand on the respective priority class, wherein the priority class isvaried on the basis of system parameters, and said interval is setshorter for higher priority classes.
 8. A method for transmitting randomaccess bursts in an apparatus for transmitting and receiving signals ina wireless telecommunication system, comprising: attempting to transmita random access burst; deciding if acknowledgement informationconfirming a successful transmission of a random access burst; settingthe length of an interval for transmitting said random access burstbased on priority classes and varying said length of interval which isset for selected priority class in accordance with a number ofunsuccessful attempts; and randomly choosing a time point for an attemptto retransmit a next random access burst from said varied length ofinterval.
 9. A method for transmitting random access bursts in anapparatus for transmitting and receiving signals in a wirelesstelecommunication system, comprising: attempting to transmit a randomaccess burst to the base station at the time point which was randomlychosen from an time interval; varying the length of said time intervalin accordance with both factors concerning priority classes and a numberof unsuccessful attempts; and randomly choosing a time point for anattempt to retransmit a next random access burst from said varied lengthof interval.
 10. A method for transmitting random access bursts in anapparatus for transmitting and receiving signals in a wirelesstelecommunication system, comprising: attempting to transmit a randomaccess burst by randomly choosing a time point from an predeterminedtime interval; deciding if said random access burst is successfullytransmitted; controlling the length of said interval by using anequation for deciding the length, wherein said equation includes aparameter indicating priority classes which is varied on the bases of anumber of unsuccessful attempts; and randomly choosing a time point foran attempt to retransmit a next random access burst from said variedlength of interval.
 11. A method for transmitting random access burstsin an apparatus for transmitting and receiving signals in a wirelesstelecommunication system, comprising: attempting to transmit a randomaccess burst by randomly choosing a time point from an predeterminedtime interval; setting the length of said interval based on the priorityclasses; varying the length of said interval corresponding to thepriority classes by using an exponential parameter indicating a numberof unsuccessful attempts; and randomly choosing a time point for anattempt to retransmit a next random access burst from said varied lengthof interval.
 12. A method for transmitting random access bursts in anapparatus for transmitting and receiving signals in a wirelesstelecommunication system, comprising: attempting to transmit a randomaccess burst by randomly choosing a time point from an predeterminedtime interval; setting the length of said interval based on the priorityclasses; varying the length of said time interval based on priorityclasses and a number of unsuccessful attempts so that the varied timewindow is exponentially increased as a function of both parametersconcerning said priority classes and said number of unsuccessfulattempts; and randomly choosing a time point for an attempt toretransmit a next random access burst from said varied length ofinterval.
 13. An apparatus for transmitting and receiving signals in awireless telecommunication system, comprising transmitting means fortransmitting a random access burst; decision means for deciding, ifacknowledgement information confirming a successful transmission of saidrandom access burst has been received; control means for randomlychoosing a time point for an attempt to retransmit said random accessburst from an interval if no acknowledgement information has beenreceived, wherein different priority classes are supported for therandom access bursts; and means for setting the length of said intervaldepending on at least two parameters, wherein a first parameterindicates the number of unsuccessful attempts to transmit a randomaccess burst and a second parameter indicates the priority class, andsaid second parameter is chosen so that said interval is set shorter forhigher priority classes.
 14. The apparatus for transmitting andreceiving signals in a wireless telecommunication system according toclaim 13, wherein said interval length is set longer when the number ofunsuccessful attempts increases.
 15. The apparatus for transmitting andreceiving signals in a wireless telecommunication system according toclaim 14, wherein said interval length depends exponentially on thenumber of unsuccessful attempts.
 16. The apparatus for transmitting andreceiving signals in a wireless telecommunication system according toclaim 13, wherein said control means randomly chooses said time pointfor a retransmission attempt after a predetermined offset time haselapsed.
 17. The apparatus for transmitting and receiving signals in awireless telecommunication system according to claim 13, wherein saidcontrol means only chooses a time point for a retransmission attempt ifit is determined that the number of unsuccessful attempts is lower thana predetermined maximum number of unsuccessful attempts.
 18. Anapparatus for transmitting and receiving signals in a wirelesstelecommunication system, comprising: transmitting means fortransmitting a random access burst; decision means for deciding ifacknowledgement information confirming a successful transmission of saidrandom access burst has been received; and control means for randomlychoosing a time point for an attempt to retransmit said random accessburst from an interval if no acknowledgement information has beenreceived, wherein different priority classes are supported for therandom access bursts, said control means setting the length of saidinterval depending on the number of unsuccessful attempts to transmit arandom access burst and on the respectively preset priority class, andsaid interval is shorter for higher priority classes.
 19. An apparatusfor transmitting and receiving signals in a wireless telecommunicationsystem, comprising: transmitting means for transmitting a random accessburst; decision means for deciding if acknowledgement informationconfirming a successful transmission of said random access burst hasbeen received; control means for randomly choosing a time point for anattempt to retransmit said random access burst from an interval if noacknowledgement information has been received, wherein differentpriority classes are supported for the random access bursts, saidcontrol means setting the length of said interval depending on thenumber of unsuccessful attempts to transmit a random access burst and onthe respective priority class, the priority class is varied on the basisof system parameters, and said interval is set shorter for higherpriority classes.
 20. An apparatus for transmitting and receivingsignals in a wireless telecommunication system, comprising: transmittingmeans for transmitting a random access burst; decision means fordeciding if acknowledgement information confirming a successfultransmission of a random access burst; and control means for setting thelength of an interval for transmitting said random access burst based onpriority classes and varying the length of said interval which is setfor a selected priority class in accordance with a number ofunsuccessful attempts, wherein said control means randomly choosing atime point for an attempt to retransmit a next random access burst fromsaid varied length of interval.
 21. An apparatus for transmitting andreceiving signals in a wireless telecommunication system, comprising:transmitting means for transmitting a random access burst to the basestation at the time point which was randomly chosen from a timeinterval; and control means for varying the length of said time intervalin accordance with both factors concerning priority classes and a numberof unsuccessful attempts, wherein said control means randomly choosing atime point for an attempt to retransmit a next random access burst fromsaid varied length of interval.
 22. An apparatus for transmitting andreceiving signals in a wireless telecommunication system, comprising:transmitting means for transmitting a random access burst by randomlychoosing a time point from an predetermined time interval; decisionmeans for deciding if said random access burst is successfullytransmitted; and control means for controlling the length of saidinterval by using an equation for deciding the length, wherein saidequation includes a parameter indicating priority classes which isvaried on the bases of a number of unsuccessful attempts, and saidcontrol means randomly choosing a time point for an attempt toretransmit a next random access burst from said varied length ofinterval.
 23. An apparatus for transmitting and receiving signals in awireless telecommunication system, comprising: transmitting means fortransmitting a random access burst by randomly choosing a time pointfrom an predetermined time interval; and control means for setting thelength of said interval based on the priority classes and for varyingthe length of said interval corresponding to the priority classes byusing an exponential parameter indicating a number of unsuccessfulattempts, wherein said control means randomly choosing a time point foran attempt to retransmit a next random access burst from said variedlength of interval.
 24. An apparatus for transmitting and receivingsignals in a wireless telecommunication system, comprising: transmittingmeans for transmitting a random access burst by randomly choosing a timepoint from an predetermined time interval; and control means for settingthe length of said interval based on the priority classes and forvarying the length of said time interval based on priority classes and anumber of unsuccessful attempts so that the varied time window isexponentially increased as a function of both parameters concerning saidpriority classes and said number of unsuccessful attempts, wherein saidcontrol means randomly choosing time point for an attempt to retransmita next random access burst from said varied length of interval.